Backlash is the term used to describe the amount of clearance between the teeth of two gears in mesh. Gear designs specify an optimum value for backlash and mass production experience identifies the variance gearsets may exhibit in each application to meet quality goals. Additionally, the positional relationship must also be controlled for economical assembly.
Backlash is an important parameter in lapping and testing procedures for gears. Lapping involves running the gear set in mesh with the application of an abrasive compound, while varying the relative position of the gears, to smoothen the tooth surfaces and modify the contact surfaces. Lapping gear sets includes the control of several machine functions simultaneously to achieve the desired effects of reduced gear noise and increased strength. A basic lapping process requirement is to control backlash near the design value to allow for smooth, unforced operation of the gear set.
Early lapping machines controlled backlash through the sequencing of machine motions. The gearset would first be meshed into direct contact condition with no backlash, called the "metal-to-metal" condition. When at this position, the machine motions would be clamped, then additional motions would mechanically actuate to separate the workpieces, producing the backlash. This process produced uncertain results because the machine elements also involved springs, motion guides with clearance, deflections, etc., which tended to be variable. Since the machine adjustment occurred at one point of mesh, the part variability which would occur at other mesh points was not considered. The availability of economical positional controls improved this situation because now the operator could compensate for certain machine errors through control inputs. Although these controls allowed for easier machine adjustments, the underlying mechanical conditions still existed.
With control of the machine motions using positional controls such as stepping motors, as disclosed in U.S. Pat. No. 3,717,958 to Ellwanger et al. for example, the opportunity now existed to provide corrective machine movements if the desired movement could be identified and quantified. Many attempts were made to apply transducers and other similar mechanical devices to sense backlash in the gearsets. The goal was to provide feedback to allow the machine motions to reset to a more ideal lapping relationship. Unfortunately, the feedback devices used were still too variable for the desired results. Measurements were still made at one mesh point, resulting in a large source of variability. Ultimately, this sort of approach was abandoned on this vintage of machine.
The advent of computer numerical controls (CNC) in lapping machines, such as disclosed in U.S. Pat. No. 4,788,476 to Ginier, provided a new level of opportunity to address the backlash situation. With direct control of machine motion position, the mechanical machine tool chassis required for lapping could be resolved into three or four machine movements. The resulting chassis was much stiffer and provided for much less variability in measurement. The conditions now existed to reapply transducers to the task of measuring backlash. The measurements, however, were still mechanical in nature and therefore subject to certain variabilities. Depending on the nature of the measurement taken, the value may also have to be correlated mathematically with defined gear backlash. Such a mathematical model in practical form is in itself a source of variability. The measurements could now be taken at several mesh positions to identify part-related variability.
It is an object of the present invention to provide a method of measuring backlash where all measurements are taken under controlled conditions whereby actual backlash values are measured, not approximated as in the prior art.